Enhanced wafer carrier

ABSTRACT

A wafer carrier used in wafer treatments such as chemical vapor deposition has pockets for holding the wafers and support surfaces for supporting the wafers above the floors of the pockets. The carrier is provided with locks for restraining wafers against upward movement away from the support surfaces. Constraining the wafers against upward movement limits the effect of wafer distortion on the spacing between the wafer and the floor surfaces, and thus limits the effects of wafer distortion on heat transfer. The carrier may include a main portion and minor portions having higher thermal conductivity than the main portion, the minor portions being disposed below the pockets.

BACKGROUND OF THE INVENTION

The present invention relates to wafer processing apparatus, to wafercarriers for use in such processing apparatus, and to methods of waferprocessing.

Many semiconductor devices are formed by epitaxial growth of asemiconductor material on a substrate. The substrate typically is acrystalline material in the form of a disc, commonly referred to as a“wafer.” For example, devices formed from compound semiconductors suchas III-V semiconductors typically are formed by growing successivelayers of the compound semiconductor using metal organic chemical vapordeposition or “MOCVD.” In this process, the wafers are exposed to acombination of gases, typically including a metal organic compound and asource of a group V element which flow over the surface of the waferwhile the wafer is maintained at an elevated temperature. One example ofa III-V semiconductor is gallium nitride, which can be formed byreaction of an organo-gallium compound and ammonia on a substrate havinga suitable crystal lattice spacing, as for example, a sapphire wafer.Typically, the wafer is maintained at a temperature on the order of500-1100° C. during deposition of gallium nitride and related compounds.

Composite devices can be fabricated by depositing numerous layers insuccession on the surface of the wafer under slightly different reactionconditions, as for example, additions of other group III or group Velements to vary the crystal structure and bandgap of the semiconductor.For example, in a gallium nitride based semiconductor, indium, aluminumor both can be used in varying proportion to vary the bandgap of thesemiconductor. Also, p-type or n-type dopants can be added to controlthe conductivity of each layer. After all of the semiconductor layershave been formed and, typically, after appropriate electric contactshave been applied, the wafer is cut into individual devices. Devicessuch as light-emitting diodes (“LEDs”), lasers, and other electronic andoptoelectronic devices can be fabricated in this way.

In a typical chemical vapor deposition process, numerous wafers are heldon a device commonly referred to as a wafer carrier so that a topsurface of each wafer is exposed at the top surface of the wafercarrier. The wafer carrier is then placed into a reaction chamber andmaintained at the desired temperature while the gas mixture flows overthe surface of the wafer carrier. It is important to maintain uniformconditions at all points on the top surfaces of the various wafers onthe carrier during the process. Minor variations in composition of thereactive gases and in the temperature of the wafer surfaces causeundesired variations in the properties of the resulting semiconductordevice. For example, if a gallium and indium nitride layer is deposited,variations in wafer surface temperature will cause variations in thecomposition and bandgap of the deposited layer. Because indium has arelatively high vapor pressure, the deposited layer will have a lowerproportion of indium and a greater bandgap in those regions of the waferwhere the surface temperature is higher. If the deposited layer is anactive, light-emitting layer of an LED structure, the emissionwavelength of the LEDs formed from the wafer will also vary. Thus,considerable effort has been devoted in the art heretofore towardsmaintaining uniform conditions.

One type of CVD apparatus which has been widely accepted in the industryuses a wafer carrier in the form of a large disc with numerouswafer-holding regions, each adapted to hold one wafer. The wafer carrieris supported on a spindle within the reaction chamber so that the topsurface of the wafer carrier having the exposed surfaces of the wafersfaces upwardly toward a gas distribution element. While the spindle isrotated, the gas is directed downwardly onto the top surface of thewafer carrier and flows across the top surface toward the periphery ofthe wafer carrier. The used gas is evacuated from the reaction chamberthrough ports disposed below the wafer carrier. The wafer carrier ismaintained at the desired elevated temperature by heating elements,typically electrical resistive heating elements disposed below thebottom surface of the wafer carrier. These heating elements aremaintained at a temperature above the desired temperature of the wafersurfaces, whereas the gas distribution element typically is maintainedat a temperature well below the desired reaction temperature so as toprevent premature reaction of the gases. Therefore, heat is transferredfrom the resistive heating element to the bottom surface of the wafercarrier and flows upwardly through the wafer carrier to the individualwafers.

Although considerable effort has been devoted in the art heretofore todesign an optimization of such systems, still further improvement wouldbe desirable. In particular, it would be desirable to provide betteruniformity of temperature across the surface of each wafer, and bettertemperature uniformity across the entire wafer carrier.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present invention provides methods of processingwafers. A method according to this aspect of the invention desirablyincludes the steps of rotating a carrier about an axis. The carrier hasa plurality of the wafers disposed thereon with top surfaces of thewafers facing in an upward direction parallel to the axis. The methoddesirably includes supporting the wafers on upwardly-facing supportsurfaces of the carrier during the rotating step and constraining thewafers against upward movement away from the support surfaces during therotating step, as well as treating the wafers during the rotating step.The treating step may include transferring heat from the carrier to thewafers. For example, a chemical vapor deposition process as discussedabove can be performed during the rotating step. The method desirablyfurther includes the step of constraining the wafers against radialmovement away from the axis during the rotating step. In preferredmethods according to this aspect of the invention, constraining thewafers against upward movement limits the effects of wafer distortion onheat transfer between the carrier and the wafers, and thus improvesuniformity of wafer surface temperature as further discussed below.

A further aspect of the invention provides wafer carriers. A wafercarrier according to this aspect of the invention desirably includes abody having oppositely-facing top and bottom surfaces, the body having aplurality of pockets open to the top surface of the body. The carrierpreferably defines an upwardly-facing support surface disposed below thetop surface of the body within each pocket. Most preferably, the carrieraccording to this aspect of the invention includes locks associated withthe pockets. Each lock desirably has a downwardly facing lock surface.When the lock is in an operative position, the lock surface extends intoor above the associated pocket so that a wafer disposed in the pocketand resting on the support surface will be at least partiallyconstrained against upward movement by the lock surface.

A wafer carrier according to a further aspect of the invention includesa body having oppositely-facing top and bottom surfaces extending inhorizontal directions and a plurality of pockets open to the topsurface, each such pocket being adapted to hold a wafer with a topsurface of the wafer exposed at the top surface of the body. The bodydesirably includes a main portion formed from a first material having afirst thermal conductivity. Preferably, the main portion hasvertically-extensive holes aligned with the pockets, and the bodyfurther includes minor portions disposed in the holes of the mainportion. The minor portions preferably are formed from a second materialhaving a second thermal conductivity higher than the first thermalconductivity. The body may further include a vertically-extensivethermal barrier between the main portion and each minor portion, thethermal barriers inhibiting conduction of heat in horizontal directionsbetween the main portion and the minor portion.

A wafer carrier according to yet another aspect of the invention mayinclude a body having main portions and minor portions, and additionallyhas a vertically-extensive border portion between the main portion andeach minor portion. The border portions desirably have thermalconductivity in the vertical direction different from the thermalconductivity of the main portion.

Yet another aspect of the invention provides treatment apparatusincorporating wafer carriers as discussed above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified, schematic sectional view depicting chemicalvapor deposition apparatus in accordance with one embodiment of theinvention.

FIG. 2 is a diagrammatic top plan view of a wafer carrier used in theapparatus of FIG. 1.

FIG. 3 is a fragmentary, diagrammatic sectional view taken along line3-3 in FIG. 2, depicting the wafer carrier in conjunction with a wafer.

FIG. 4 is a fragmentary top plan view depicting a portion of the wafercarrier of FIGS. 2 and 3.

FIG. 5 is a fragmentary view on an enlarged scale depicting the regionindicated in FIG. 4.

FIG. 6 is a view similar to FIG. 3 but depicting the wafer carriers andwafer of FIGS. 1-5 during a different operating condition.

FIG. 7 is a view similar to FIG. 7 but depicting a conventional wafercarrier and wafer in an operating condition similar to that of FIG. 7.

FIG. 8 is a fragmentary top plan view depicting a wafer carrieraccording to a further embodiment of the invention.

FIG. 9 is a fragmentary sectional view on an enlarged scale taken alongline 9-9 in FIG. 8.

FIG. 10 is a fragmentary sectional view on an enlarged scale taken alongline 10-10 in FIG. 8.

FIGS. 11, 12, and 13 are fragmentary, diagrammatic sectional viewsdepicting portion of wafer carriers in accordance with furtherembodiments of the invention.

DETAILED DESCRIPTION

Chemical vapor deposition apparatus in accordance with one embodiment ofthe invention includes a reaction chamber 10 having a gas distributionelement 12 arranged at one end of the chamber. The end having the gasdistribution element 12 is referred to herein as the “top” end of thechamber 10. This end of the chamber typically, but not necessarily, isdisposed at the top of the chamber in the normal gravitational frame ofreference. Thus, the downward direction as used herein refers to thedirection away from the gas distribution element 12; whereas the upwarddirection refers to the direction within the chamber, toward the gasdistribution element 12, regardless of whether these directions arealigned with the gravitational upward and downward directions.Similarly, the “top” and “bottom” surfaces of elements are describedherein with reference to the frame of reference of chamber 10 andelement 12.

Gas distribution element 12 is connected to sources 14 of gases to beused in the CVD process, such as a carrier gas and reactant gases suchas a source of a group III metal, typically a metalorganic compound, anda source of a group V element as, for example, ammonia or other group Vhydride. The gas distribution element is arranged to receive the variousgases and direct a flow of gasses generally in the downward direction.The gas distribution element 12 desirably is also connected to a coolantsystem 16 arranged to circulate a liquid through the gas distributionelement so as to maintain the temperature of the element at a desiredtemperature during operation. Chamber 10 is also equipped with anexhaust system 18 arranged to remove spent gases from the interior ofthe chamber through ports (not shown) at or near the bottom of thechamber so as to permit continuous flow of gas in the downward directionfrom the gas distribution element.

A spindle 20 is arranged within the chamber so that the central axis 22of the spindle extends in the upward and downward directions. Thespindle has a fitting 24 at its top end, i.e., at the end of the spindleclosest to the gas distribution element 12. In the particular embodimentdepicted, the fitting 24 is a generally conical element. Spindle 20 isconnected to a rotary drive mechanism 26 such as an electric motordrive, which is arranged to rotate the spindle about axis 22. A heatingelement 28 is mounted within the chamber and surrounds spindle 20 belowfitting 24. The chamber is also provided with an openable port 30 forinsertion and removal of wafer carriers. The foregoing elements may beof conventional construction. For example, suitable reaction chambersare sold commercially under the registered trademark TURBODISC by VeecoInstruments, Inc. of Plainview, N.Y., USA, assignee of the presentapplication.

In the operative condition depicted in FIG. 1, a wafer carrier 32 ismounted on the fitting 24 of the spindle. The wafer carrier has astructure which includes a body generally in the form of a circular dischaving a central axis 25 extending perpendicular to the top and bottomsurfaces. The body of the wafer carrier has a first major surface,referred to herein as the “top” surface 34, and a second major surface,referred to herein as the “bottom” surface 36. The structure of thewafer carrier also has a fitting 39 arranged to engage the fitting 24 ofthe spindle and to hold the body of the wafer carrier on the spindlewith the top surface 34 facing upwardly toward the gas distributionelement 12, with the bottom surface 36 facing downwardly toward heatingelement 28 and away from the gas distribution element. Merely by way ofexample, the wafer carrier body may be about 465 mm in diameter, and thethickness of the carrier between top surface 34 and bottom surface 32may be on the order of 15.9 mm. In the particular embodimentillustrated, the fitting 39 is formed as a frustoconical depression inthe bottom surface of the body 32. However, as described in copending,commonly assigned US Patent Publication No. 2009-0155028 A1, thedisclosure of which is hereby incorporated by reference herein, thestructure may include a hub formed separately from the body and thefitting may be incorporated in such a hub. Also, the configuration ofthe fitting will depend on the configuration of the spindle.

The body desirably includes a main portion 38 formed as a monolithicslab of a non-metallic refractory first material as, for example, amaterial selected from the group consisting of silicon carbide, boronnitride, boron carbide, aluminum nitride, alumina, sapphire, quartz,graphite, and combinations thereof, with or without a refractory coatingas, for example, a carbide, nitride or oxide.

The body of the carrier defines a plurality of circular pockets 40 opento the top surface. As best seen in FIGS. 1 and 3, the main portion 38of the body defines a substantially planar top surface 34. The mainportion 38 has holes 42 extending through the main portion, from the topsurface 34 to the bottom surface 36. A minor portion 44 is disposedwithin each hole 42. The minor portion 44 disposed within each holedefines a floor surface 46 of the pocket 40, the floor surface beingrecessed below the top surface 34. The minor portions 44 are formed froma second material, preferably a non-metallic refractory materialconsisting of silicon carbide, boron nitride, boron carbide, aluminumnitride, alumina, sapphire, quartz, graphite, and combinations thereof,with or without a refractory coating as, for example, a carbide, nitrideor oxide. The second material desirably is different from the firstmaterial constituting the main portion. The second material mostpreferably has a thermal conductivity higher than the thermalconductivity of the first material. For example, where the main portionis formed from graphite, the minor portions may be formed from siliconcarbide. The minor portions 44 and the main portion 38 cooperativelydefine the bottom surface 36 of the body. In the particular embodimentdepicted in FIG. 3, the bottom surface of the main portion 38 is planar,and the bottom surfaces of the minor portions 44 are coplanar with thebottom surface of the main portion, so that the bottom surface 36 isplanar.

The minor portions 44 are frictionally engaged with the walls of theholes 40. For example, the minor portions may be press-fit into theholes, or shrink-fitted by raising the main portion to an elevatedtemperature and inserting cold minor portions into the holes. Desirably,all of the pockets are of uniform depth. This uniformity can be achievedreadily by forming all of the minor portions to a uniform thickness as,for example, by grinding or polishing the minor portions.

There is a thermal barrier 48 between each minor portion 44 and thesurrounding material of the main portion 38. The thermal barrier is aregion having thermal conductivity in the horizontal directions,parallel to the top and bottom surfaces of the carrier, which is lowerthan the thermal conductivity of the bulk material of the main portion.In the particular embodiment depicted in FIG. 3, the thermal barrierincludes a macroscopic gap 48, as, for example, a gap about 100 micronsor more thick, formed by a groove in the wall of the main portion 38defining the hole 42. This gap contains a gas such as air or the processgasses encountered during operation, and hence has much lower thermalconductivity than the neighboring solid materials.

The abutting surfaces of the minor portions 44 and main portion 38 alsodefine parts of the thermal barrier. Although these surfaces abut oneanother on a macroscopic scale, neither surface is perfectly smooth.Therefore, there will be microscopic, gas-filled gaps between parts ofthe abutting surfaces. These gaps will also impede thermal conductionbetween the minor portion 44 and main portion 38.

As best seen in FIGS. 3 and 4, the carrier further includes locks 50associated with the pockets. The locks 50 preferably are formed from arefractory material having thermal conductivity which is lower than theconductivity of the minor portions 44 and preferably lower than theconductivity of the main portion 38. For example, the locks may beformed from quartz. Each lock includes a middle portion 52 (FIG. 3) inthe form of a vertical cylindrical shaft and a bottom portion 54 in theform of a circular disc coaxial with the middle portion and projectingoutwardly away from the axis of the middle portion. The bottom portionof each lock defines an upwardly-facing support surface 56. Each lockfurther includes a top portion 58 projecting transverse to the axis ofthe middle portion. The top portion is not symmetrical about the axis ofthe middle portion 52. The top portion 58 of each lock defines adownwardly-facing lock surface 60 overlying the support surface 56 ofthe lock but spaced apart from the support surface. Thus, each lockdefines a gap 62 between surfaces 56 and 60.

Each lock is secured to the wafer carrier. As best seen in FIGS. 3 and5, the middle portion 52 of each lock lies against the wall of the hole42 in the main portion. The bottom portion 54 extends into an undercut64 (FIG. 3) in the wall of the hole, so that the lock is retainedagainst vertical movement relative to the body of the wafer carrier, andso that the bottom portion 50 rests on the floor surface 46 of thepocket. As seen in FIG. 5, the main portion 38 may have projections 66extending into the pocket from the wall of hole 42 so as to retain thelock against movement in horizontal directions.

With the locks in the operative position shown in FIGS. 3 and 4, the topportion 58 of each lock projects inwardly, toward the center 68 of thepocket. Each lock can be turned to an inoperative position in which thetop portion is rotated as depicted in broken lines in FIG. 5 at 58′, sothat the top portion does not project inwardly toward the center of thepocket.

Three locks 50 are provided for each pocket 50. Lock 50 a, referred toherein as an “inner” lock, is disposed at a location lying at a distanceD_(50A) (FIG. 4) from the central axis 25 of the carrier body which isless than the distance D_(c) from the center 68 of the pocket to thecentral axis 25. Locks 50 b and 50 c are “outer” locks, which aredisposed at distances from the central axis 25 of the carrier greaterthan the distance D_(c) from the central axis to the center 68 of thepocket. In the particular arrangement depicted, the locks are spacedapart from one another at around the periphery of the pocket 40, withequal spaces between adjacent locks. The inner lock 50 a lies on aradial line R extending through the central axis 25 of the carrier andthe center 68 of the pocket, whereas the two outer locks 50 b and 50 care disposed on opposite sides of this radial line.

In operation, the carrier is loaded with circular, disc-like wafers 70.With one or more of the locks 50 associated with each pocket in itsinoperative position, the wafer is placed into the pocket so that abottom surface 72 of the wafer rests on the support surfaces 56 of thelocks. The support surfaces of the locks cooperatively support thebottom surface 72 of the wafer above the floor surface 46 of the pocket,so that there is a gap 73 (FIG. 3) between the bottom surface of thewafer and the floor surface of the pocket, and so that the top surface74 of the wafer is coplanar or nearly coplanar with the top surface 34of the carrier. The dimensions of the carrier, including the locks, areselected so that there is a very small clearance between the edge orperipheral surface 76 of the wafer and the middle portions 52 of thelocks. The middle portions of the locks thus center the wafer within thepocket, so that the distance D_(w) between the edge of the wafer and thewall of the pocket is substantially uniform around the periphery of thewafer.

The locks are brought to the operative positions, so that the topportion 58 of each lock, and the downwardly facing lock surface 60 (FIG.3) projects inwardly over the pocket and hence over the top surface 74of the wafer. The lock surfaces 60 are disposed at a vertical levelhigher than the support surfaces 56. Thus, the wafer is engaged betweenthe support surfaces 56 and the lock surfaces, and constrained againstupward or downward movement relative to the carrier. The top and bottomelements of the locks desirably are as small as practicable, so thatthese elements contact only very small parts of the wafer surfacesadjacent the periphery of each wafer. For example, the lock surfaces andsupport surfaces may engage only a few square millimeters of the wafersurfaces.

Typically, the wafers are loaded onto the carrier while the carrier isoutside of the reaction chamber. The carrier, with the wafers thereon,is loaded into the reaction chamber using conventional robotic apparatus(not shown), so that the fitting 39 of the carrier is engaged with thefitting 24 of the spindle, and the central axis 25 of the carrier iscoincident with the axis 22 of the spindle. The spindle and carrier arerotated about this common axis. Depending on the particular processemployed, such rotation may be at hundreds of revolutions per minute ormore.

The gas sources 14 are actuated to supply process gasses and carriergasses to the gas distribution element 12, so that these gasses flowdownwardly toward the wafer carrier and wafers, and flow generallyradially outwardly over the top surface 34 of the carrier and over theexposed top surfaces 74 of the wafers. The gas distribution element 12and the walls of chamber 10 are maintained at relatively lowtemperatures to inhibit reaction of the gasses at these surfaces.

Heater 28 is actuated to heat the carrier and the wafers to the desiredprocess temperature, which may be on the order of 500 to 1200° C. forcertain chemical vapor deposition processes. Heat is transferred fromthe heater to the bottom surface 36 of the carrier body principally byradiant heat transfer. The heat flows upwardly by conduction through themain portion 38 of the carrier body to the top surface 34 of the body.Heat also flows upwardly through the minor portions 44 of the wafercarrier, across the gaps 73 between the floor surfaces of the pocketsand the bottom surfaces of the wafers, and through the wafers to the topsurfaces 74 of the wafers. Heat is transferred from the top surfaces ofthe body and wafers to the walls of chamber 10 and to the gasdistribution element 12 by radiation, and is also transferred to theprocess gasses.

The process gasses react at the top surfaces of the wafers to treat thewafers. For example, in a chemical vapor deposition processes, theprocess gasses form a deposit on the wafer top surfaces. Typically, thewafers are formed from a crystalline material, and the depositionprocess is epitaxial deposition of a crystalline material having latticespacing similar to that of the material of the wafer.

For process uniformity, the temperature of the top surface of each wafershould be constant over the entire top surface of the wafer, and equalto the temperature of the other wafers on the carrier. To accomplishthis, the temperature of the top surface of 74 of each wafer should beequal to the temperature of the carrier top surface 34. The temperatureof the carrier top surface depends on the rate of heat transfer throughthe main portion 38 of the body, whereas the temperature of the wafertop surface depends on the rate of heat transfer through the minorportion 44, the gap 73 and the wafer itself. The high thermalconductivity, and resulting low thermal resistance, of the minorportions 44 compensates for the high thermal resistance of the gaps 73,so that the wafer top surfaces are maintained at temperaturessubstantially equal to the temperature of the carrier top surface. Thisminimizes heat transfer between the edges of the wafers and thesurrounding portions of the carrier and thus helps to maintain a uniformtemperature over the entire top surface of each wafer. To provide thiseffect, the floor surfaces of the pockets 46 must be at a highertemperature than the adjacent parts of the main portion 38. The thermalbarriers 48 between the minor portions 44 and the main portion 38 of thebody minimize heat loss from the minor portions 44 to the main portion,and thus help to maintain this temperature differential.

During operation, each wafer tends to move away from the central axis 25of the carrier due to the centrifugal forces caused by rotation of thecarrier. Each wafer is maintained precisely centered in the pocket bythe middle portions 52 of the locks. The centrifugal forces urge eachwafer against the middle portions 52 of the outer locks 50 b and 50 c.These portions act as abutment elements, which restrain the waferagainst outward movement. The precise centering of the wafer maintainsthe uniform distance D_(w) between the edge of the wafer and thesurrounding wall of the pocket, and avoids direct contact between thewafer and the pocket wall. This minimizes heat transfer between thewafer and the carrier, and also helps to assure that any heat transferwhich does occur is substantially radially symmetrical about the centerof the wafer.

During operation, the wafer may distort from a flat disc to a domedshape. For example, epitaxial deposition of a crystalline materialhaving an undistorted lattice spacing slightly different from theundistorted lattice spacing of the crystalline material of the waferapplies a tensile or compressive stress at the top surface of the wafer,and the wafer distorts to relieve such stress. FIG. 6 depicts the samewafer and pocket as shown in FIG. 3, with the wafer 70 distorted to adome-like shape. Such distortion causes the center of the wafer to bowtowards or away from the floor surface 46 of the pocket, and thus causesthe height of the gap 73 between the wafer bottom surface 72 and floorsurface 46 to vary. With the edges of the wafer restrained againstupward movement by the locks 50, the difference in height ΔH of the gapis relatively small; it is given by the formula:ΔH=K*d ²/8

Where:

K is the wafer curvature; and

d is the diameter of the wafer.

In a conventional wafer carrier shown in FIG. 7, the pocket has anundercut peripheral wall 142, and a circular support surface 156. Thewafer comes to rest against an outer portion 142 a of the peripheralwall furthest from the central axis of the carrier. The undercutperipheral wall holds the outer part 101 of the wafer down against thesupport surface 156. However, the inner part 103 of the wafer, closestto the central axis of the carrier, is not restrained against upwardmovement relative to the carrier, so that the curvature of the wafercauses the inner part 103 to lift upwardly, away from the supportsurface 156. This causes a large difference in the height ΔH′ of the gap173 between the bottom surface 172 of the wafer and the floor surface146 of the pocket. Using the conventional wafer carrier:ΔH′=K*d ²/2.

Stated another way, the difference ΔH with the edge of the waferrestrained by the locks (FIG. 3) is only one-fourth the difference ΔH′with the conventional carrier. Because the rate of heat transfer acrossthe gap varies directly with the height of the gap, the dramaticreduction in the difference in gap height provides a correspondingreduction in differences in heat transfer to various parts of the wafer.Moreover, with the wafer restrained by the locks (FIG. 3) the height ofthe gap, and hence the heat transfer, vary in a pattern which isradially symmetrical about the center of the wafer. Because the innerpart of the wafer edge does not lift up when the wafer is restrained,there will be no disturbance in the flow of gasses across the wafercarrier and wafer top surfaces caused by edges of the wafer projectingabove the top surface of the carrier.

The locks themselves may cause small, localized disturbances in the gasflow. This phenomenon is minimized by making the locks, and particularlythe top portions 58 of the locks, as small as possible and as thin aspossible. Also, the top portions of the locks preferably havestreamlined shapes. There will be some minor heat transfer by conductionthrough the locks, but this effect is limited by the small areas ofcontact between the locks and the wafer, and by the low thermalconductivity of the locks.

The configuration discussed above can be varied. For example, locks asdiscussed above can be used with a wafer carrier having a unitary bodywithout the minor portions discussed above. Also, the configuration ofthe locks, support surfaces and abutment elements can be varied fromthat discussed above. The wafer carrier depicted in FIGS. 8-10 has aunitary body 232 defining pockets 240. An upwardly-facing supportsurface within each pocket is defined by a plurality of small supportelements 254 in the form of disc-like buttons resting on the floorsurface 246 of the pocket. These support elements are distributed aroundthe periphery of the pocket.

Each pocket also has a lock 250. The lock is slidably mounted to thecarrier body for movement in directions towards and away from thecentral axis 225 of the carrier. The lock has a wafer-engaging surface260 (FIGS. 8 and 10) which slopes away from the center 268 of the pocketin the downward direction. Stated another way, the lower portion ofsurface 268 lies further from the center 268 of the pocket and closer tothe central axis 225 of the carrier than the upper portion of the samesurface. Thus, surface 260 faces downwardly toward the floor surface 246as well as inwardly toward the center of the pocket. The carrier has achannel 202 which, as seen in cross-section in FIG. 9, has a dovetail orgenerally trapezoidal shape. Lock 250 has a corresponding shape. Thelock is engaged in the channel so that the lock can move between theinoperative position shown in broken lines at 250′ and the operativeposition shown in solid lines. In the operative position, the end of thelock with engaging surface 260 projects into the pocket and beyond thevertical wall 242 of the pocket so that surface 268 bears on the upperedge of a wafer 270 received in the pocket. The centrifugal force causedby rotation of the carrier urges the lock away from the central axis 225and thus toward the center 268 of the pocket. Thus, as the carrierrotates, lock 250 holds the inner part 253 of the wafer down and forcesthe wafer into engagement with the supports 254. The dimensions of thelock are exaggerated for clarity of illustration. In practice, thoseparts of the lock which contact the wafer should be as small aspracticable to minimize heat transfer through the lock.

Each pocket also has abutment elements 252. The abutment elements aredisposed at distances further from the central axis 225 of the carrierthan the center 268 of the pocket. The abutment elements have surfaces269 sloping away from the central axis 225 in the downward direction. Inoperation, centrifugal forces on the wafer tend to force the waferagainst surfaces 269, so that the abutment elements hold the outer part251 of the wafer down against supports 254. The abutment elements may beformed separately from the carrier body or may be integral with thecarrier body.

In a further variant (FIG. 11), minor portions 344 of the carrier bodymay be mounted to the main portion 338 by bushings 348 formed fromquartz or another material having thermal conductivity lower than theconductivities of the main portion and minor portions. Here again, theminor portion desirably has higher thermal conductivity than the mainportion. The bushing serves as part of the thermal barrier between theminor portion and main portion. The solid-to-solid interfaces betweenthe bushing and minor portion, and between the bushing and main portion,provide additional thermal barriers. In this variant, the bushingdefines the vertical wall 342 of the pocket.

The embodiment of FIG. 12 is similar to the embodiment discussed abovewith reference to FIGS. 1-6, except that each minor portion 444 includesa body 443 of smaller diameter than the corresponding hole 442 in themain portion 438, so that a gap 448 is provided as a thermal barrier.Each minor portion also includes a head 445 closely fitted in the mainportion 438 to maintain concentricity of the minor portion and the hole442.

The wafer carrier of FIG. 13 includes a main portion and minor portions544 similar to the carrier discussed above with reference to FIGS. 1-6.However, the carrier body of FIG. 13 includes ring-like border portions502 encircling the minor portions and disposed between each minorportion and the main portion. The border portions 502 have thermalconductivity different from the thermal conductivity of the main portionand minor portions. As illustrated, the border portions are alignedbeneath the periphery of each pocket. In a further variant, the borderportions may be aligned beneath a part of the top surface 534surrounding each pocket. The thermal conductivity of the border portionscan be selected independently to counteract heat transfer to or from theedges of the wafers. For example, where those portions of the topsurface 534 tend to be hotter than the wafer, the thermal conductivityof the border portions can be lower than the conductivity of the mainportion.

The wafer carriers and apparatus discussed above can materially reducetemperature differences across the surfaces of the wafers. However, evenwith the features discussed above, some temperature non-uniformity canoccur. Because the temperature distribution is generally radiallysymmetrical about the center of each wafer, other measures which tend tosuppress temperature differences can be applied readily. For example, asdisclosed in co-pending, commonly assigned US Patent ApplicationPublication No. 2010-0055318, the disclosure of which is herebyincorporated by reference herein, the thermal conductance of the wafercarrier can be varied by varying its thickness. For example, where thewafer tends to bow toward the floor surface of the pocket at the centerof the pocket as shown in FIG. 6, the thermal conductance of the gap atthe center of the pocket will be higher than the thermal conductance ofthe gap near the edge of the pocket. This can be counteracted byincreasing the thickness of the carrier body in the region of the bodybeneath the center of the pocket, so as to reduce the thermalconductance in this area.

As these and other variations and combinations of the features describedabove can be utilized, the foregoing description of the preferredembodiments should be taken as illustrating, rather than limiting, thescope of the invention.

The invention claimed is:
 1. A wafer carrier comprising a body havingoppositely-facing top and bottom surfaces extending in horizontaldirections and a plurality of pockets open to the top surface, each suchpocket being adapted to hold a wafer with a top surface of the waferexposed at the top surface of the body, the body including a mainportion formed from a first material having a first thermalconductivity, the main portion having vertically-extensive holes alignedwith the pockets, the body further including minor portions formed froma second material having a second thermal conductivity higher than thefirst thermal conductivity, the minor portions being disposed in theholes of the main portion, the body having a vertically-extensivethermal barriers between the main portion and each minor portion, thethermal barriers inhibiting conduction of heat in horizontal directionsbetween the main portion and the minor portion.
 2. A wafer carrier asclaimed in claim 1 wherein the minor portions define floor surfaces ofthe pockets recessed below the top surface, the minor portions alsodefining parts of the bottom surface.
 3. A wafer carrier as claimed inclaim 1 wherein the thermal barriers include interfaces between abuttingsurfaces of the minor portions and the main portions.
 4. A wafer carriercomprising a body having oppositely-facing top and bottom surfacesextending in horizontal directions and a plurality of pockets open tothe top surface, each such pocket being adapted to hold a wafer with atop surface of the wafer exposed at the top surface of the body, thebody including a main portion having vertically-extensive holes alignedwith the pockets, the body further including minor portions disposed inthe holes of the main portion, the body having a vertically-extensiveborder portion between the main portion and each minor portion, theborder portions having thermal conductivity in the vertical directiondifferent from the thermal conductivity of the main portion.
 5. A wafercarrier as claimed in claim 4 wherein the minor portions have thermalconductivity different from the main portion and different from theborder portions.
 6. A wafer carrier as claimed in claim 4 wherein thepockets are circular.